The invention relates to the field of analog instrumentation and particularly to circuits used in calibrating analog instruments to correctly respond when the analog signal being measured is zero.
In analog instruments such as instruments for quantitative photometric analysis, the level of a signal such as the photometric signal is calibrated at a zero level (zero percent transmittance) and at a 100% level (100% transmittance) and the accuracy of the instrument is dependent on being able to accurately indicate both the zero and the 100% level. To achieve the desired accuracy, the instrument typically includes a calibration circuit which simulates the signal at the zero and 100% level and includes circuit calibration controls to adjust the display output to indicate the correct input magnitude and polarity at these levels.
Correctly calibrating an instrument is typically more critical at the zero level than at the 100% level because a small calibration error at the zero level may contribute an error which is substantial as compared to the quantity actually being measured. For example, if a typical transmittance measurement instrument were calibrated by an operator by simply adjusting the calibration controls until the instrument output reads zero to correspond to zero percent transmittance (T), the instrument may actually be responding to a signal between -0.07%T and +0.07%T. Accordingly, the zero indication for the instrument can easily be set improperly so that subsequent measurement near the zero % transmittance level are subject to being inaccurate due to the failure to properly adjust the instrument to indicate zero when in actuality the input was zero.
Typically, this problem in prior analog instruments has been addressed by providing high gain amplifiers ahead of a comparator circuit to amplify the analog signal so that even a small analog voltage can be indicated as being other than zero by a comparator. The difficulty with this approach, however, is that the operator has no indication during adjustment of the calibrating control as to how close the zero point really is. The only indication provided by such circuits is the abrupt change in the digital state at the comparator output which occurs after the calibration control has adjusted the instrument through zero. As the only change is an abrupt change, the operator may acutally have advanced the calibrating control beyond the zero point before he can react to the change in output state. As such, accurate calibration at the zero level is very difficult to achieve by this method. Analog meters such as electro mechanical volt meters can be used where low accuracy is acceptable but where more precise measurement is required, they cannot be used to calibrate an instrument at the zero point. Analog to digital instruments such a A/D volt meters with many digits can be used but the cost is frequently prohibitive.